Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752471AbbBXUUg (ORCPT ); Tue, 24 Feb 2015 15:20:36 -0500 Received: from mail-wi0-f171.google.com ([209.85.212.171]:42231 "EHLO mail-wi0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750925AbbBXUUe (ORCPT ); Tue, 24 Feb 2015 15:20:34 -0500 MIME-Version: 1.0 In-Reply-To: <54ECD6BA.9070009@hurleysoftware.com> References: <1424795830-31223-1-git-send-email-peter@hurleysoftware.com> <54ECD6BA.9070009@hurleysoftware.com> From: Rob Herring Date: Tue, 24 Feb 2015 14:20:12 -0600 X-Google-Sender-Auth: FLxDfYCEllbqdtbT7PNBCxtzZM0 Message-ID: Subject: Re: [PATCH -next 00/13] Extensible console matching & direct earlycon To: Peter Hurley Cc: Greg Kroah-Hartman , Andrew Morton , Arnd Bergmann , Jiri Slaby , "linux-serial@vger.kernel.org" , "linux-kernel@vger.kernel.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3803 Lines: 84 On Tue, Feb 24, 2015 at 1:53 PM, Peter Hurley wrote: > Hi Rob, > > On 02/24/2015 02:27 PM, Rob Herring wrote: >> On Tue, Feb 24, 2015 at 10:36 AM, Peter Hurley wrote: >>> Hi Greg & Andrew, >>> >>> This patch series implements: >>> 1. console-definable (aka extensible) matching >>> 2. generic earlycon-to-console handoff via extensible matching >>> 3. arch/prom support for direct earlycon >>> >>> >>> Extensible console matching >>> >>> Extensible console matching enables the console itself to define the >>> conditions for a "command line" match. This mimics the design of >>> device matching in the driver model. Two important use-cases which this >>> feature enables are generic earlycon-to-console handoff and support >>> for driver migration. >>> >>> Earlycon-to-console handoff was implemented in 2007. Console command >>> lines of the form: >>> console=uart,io,0x3f8,115200n8 >>> start an earlycon and later allow the 8250 driver console to takeover. >>> Unfortunately this implementation requires direct coupling between >>> the earlycon and the console and is facilitated by ugly hacks like >>> editing the console command line in-place. >>> >>> Extensible console matching allows the 8250 driver to directly match >>> that console command line instead, and enables other serial drivers >>> to trivially support console handoff themselves. >>> >>> In addition, extensible console matching allows a new driver to >>> provide support for a different driver's console. This requirement >>> stems from needing to minimize breakage when migrating serial drivers. >>> Since many devices are based on the original 8250/16550 designs >>> but sometimes have features incompatible with the existing 8250 driver >>> support, the initial driver is often standalone. When/if the standalone >>> driver is migrated to the 8250 driver, the problem of console names in >>> the command line remains. Extensible console matching enables a simple >>> migration path. >>> >>> >>> Direct earlycon >>> >>> This feature enables arches and proms to start an earlycon directly, >>> rather than requiring an "earlycon=" command line parameter. >>> Devicetree can already do this via the 'linux,stdout-path' property, >>> but arch and prom code requires direct coupling to the serial driver. >>> >>> This support is implemented by judicious refactoring and the same >>> construct that devicetree and early_param use: a link table containing >>> the necessary information (name and setup() function) to find and >>> bind the appropriate earlycon "driver". >> >> I've skimmed thru this and it looks like a great improvement. >> >> One problem we have currently with DT stdout-path and earlycon is a >> preferred console does not get registered, so the console will get >> switched to tty0 and you lose your console. The problem is DT does not >> know the console name to register a preferred console. It looks like >> this series may help this problem, but I'm not sure and wanted your >> thoughts. > > I thought that of_alias_scan() + of_console_check() caused DT stdout-path > to add_preferred_console() the driver console @ port registration time > via uart_add_one_port() -> of_console_check(). > > Is that not how that works? Yes, I believe that is how it works with earlycon not enabled. This doesn't work when earlycon is enabled with just "earlycon" on the command line. The fix I have is here[1], but I don't like putting DT specifics into the console code. Rob [1] https://lkml.org/lkml/2014/7/25/333 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/